This invention relates to stereo receivers for stereo radio broadcasts, and to apparatus for processing stereo representative signals in general.
On Mar. 4, 1982 the Federal Communications Commission (FCC) adopted a revolutionary Report and Order authorizing the so-called "marketplace approach" to the selection of a national standard for the transmission of stereophonic radio programs in the AM broadcast band (535-1605 kilohertz). Since then, several different AM stereo systems have been competing in the marketplace.
In the various AM stereo systems which have entered the marketplace, the left channel (L) and right channel (R) audio signals are added together to form an L+R signal, which is used to amplitude modulate the radio-frequency (RF) carrier in the usual manner. By subtracting the R from the L signal, an L-R signal is formed which is used to phase modulate (PM) the RF carrier in some AM stereo systems or, to quadrature modulate (QM) the carrier in others.
The transmitted signals from all AM stereo systems which have been and are currently being used in the United States are compatible with monaural AM receivers, but none are compatible with each other.
Because of this mutual incompatibility among the various systems, receiver manufacturers have been faced with the choice of designing AM stereo receivers for one system only (so-called "single-system" receivers), or receivers capable of stereophonic reception of signals of more than one of the systems being broadcast (so-called "multi-system" receivers). Both types of receivers are available in the marketplace.
Multi-system receivers have been of two general types. Some include pilot-signal detection circuits which provide automatic switching of those receiver circuits required to properly decode the particular type of AM stereo signal being received. Other multi-system receivers are equipped with manual switches for that purpose.
Among circuits that have been switched in multi-system receivers are: distortion-correction circuits, 90.degree. phase-difference networks used in one of the AM stereo systems (the Kahn/Hazeltine system), limiters, and detectors. It is, therefore, an object of the invention is to provide a multi-system or "universal" AM stereo receiver which requires no circuit switching, manual or automatic, in the stereo decoder, but which provides stereophonic reception of broadcasts that use any of the different AM stereo systems which remain in the marketplace. It is a further object of the invention to provide apparatus for processing stereo signals to produce enhanced perception of stereo separation, such as in an AM stereo receiver. A further object is to provide a low cost universal AM stereo receiver.